a(n) = (8^n - 1)*3/7.

Terms

a(0) =0a(1) =3a(2) =27a(3) =219a(4) =1755a(5) =14043a(6) =112347a(7) =898779a(8) =7190235a(9) =57521883a(10) =460175067a(11) =3681400539a(12) =29451204315a(13) =235609634523